The present invention relates to a horizontal S-distortion correcting circuit suited profitably for use in a multi-scanning display device.
In a conventional high-definition CRT (cathode ray tube) display device, there is used a deflecting yoke of a uniform field type which scarcely brings about defocus in deflection (distortion of picture a displayed). In that case, geometrical relation between the scanning locus of an electron beam and the plane of a screen of the picture tube is such that the sine value of deflection angle is in proportion to a deflecting current while deflection distance on the screen is proportional to the tangent of the deflection angle. Consequently, when the deflecting current is of a saw tooth wave suffering no distortion, there rather makes appearance such geometrical distortion on the display screen that a peripheral portion of a picture is excessively deflected or distorted in the peripheral direction in proportion to the third power of the deflection angle. The geometrical distortion mentioned above is generally classified into a so-called pincushion distortion and S distortion (spiral distortion) which are compensated, respectively, by various appropriate means known in the art.
The present invention is concerned with correction of the horizontal S distortion among others.
The multi-scan display device is capable of handling a plurality of different horizontal frequencies. By way of example, three different horizontal scanning frequencies of 16 KHz, 32 KHz and 64 KHz can selectively be used in one and the same display device.
As a known means for compensating for the S distortion, there can be mentioned a S capacitor (i.e. S-distortion correcting capacitor) connected in cascade to a horizontal deflection coil. However, the S capacitor can not satisfactorily correct the S distortion except for that making appearance at a particular horizontal scanning frequency, because the S capacitor is of a fixed capacitance value. More specifically, the quantity for the correction increases in proportion to the second power of the horizontal scanning frequency f.sub.H . Accordingly, when capacitance of the S capacitor is so selected that the correction quantity of a proper value can be obtained at the center value of the horizontal scanning frequency, then the correction quantity as selected naturally becomes insufficient at lower horizontal scanning frequencies while excessive correction will become effective in a higher range of the horizontal scanning frequencies. In this manner, in the multiscan display device, only the single S-distortion correcting capacitor is ineffective for correcting to any satisfactory extent the S distortion over all the horizontal scanning frequencies.
Under the circumstances, in the multi-scan display device, there are provided a plurality of S capacitors having different capacitances in correspondence to a variety of horizontal scanning frequencies, respectively, wherein S capacitance is changed over from one to another value correspondingly upon every change of the horizontal scanning frequency.
Needless to say, incorporation of the plural S capacitors in the horizontal deflecting circuit involves increase in the expenditure and the scale as well as complication of the circuit and exerts adverse influence to the reliability in operation of the circuit as well. Thus, it is desirable to be capable of correcting the S distortion completely over the whole range of the horizontal scanning frequencies by using only a single capacitor of a fixed capacitance value.